1. Field of the Invention
The present invention relates to a lubricant to be used in a magnetic recording medium or the like, a recording medium using the lubricant, and a carboxylic acid type compound constituting the lubricant.
2. Description of the Related Art
As for a conventional recording medium, for example, a magnetic recording medium, a so-called metallic thin film type magnetic recording medium in which a ferromagnetic metallic material is deposited on a non-magnetic supporting body by a technique of evaporation or the like to allow the ferromagnetic metallic material to be a magnetic layer, or a so-called coating type magnetic recording medium in which a magnetic coating material comprising extremely fine magnetic particles and a resin binder is applied on a non-magnetic supporting body to be a magnetic layer has been known.
These conventional magnetic recording media have a substantially large contact area against a sliding member such as a magnetic head or a guide roller, since a surface of the magnetic layer thereof is extremely smooth, a friction coefficient becomes large and, then, an adhesion phenomenon (so-called adherence) tends to be generated; therefore, there are many problems in that they are insufficient in a traveling property, durability and the like.
Further, since a metal is used in the magnetic layer, the magnetic layer is corroded with passage of time due to a high-temperature, high-humidity environment or presence of a corrosive gas and, accordingly, there is a risk of impairing reliability of data storage.
In order to improve these problems, applications of various types of lubricants have been studied and many trials for suppressing friction and preventing corrosion of the magnetic layer by allowing higher fatty acids, esters thereof or the like to be internally added in the magnetic layer of the magnetic recording medium or applied thereon as a topcoat have been conducted in the past.
However, from the standpoint of properties of the lubricant to be used in the magnetic recording medium or the like, extremely severe characteristics are required in the lubricant. Namely, the lubricant to be used in the magnetic recording medium is required to be:
(1) excellent in low temperature characteristics in order to secure a predetermined lubricating effect when it is used in a cold district;
(2) capable of being applied extremely thinly due to a problem of spacing between a magnetic head and, simultaneously, exerting sufficient lubricating characteristics;
(3) durable for a long time (period) usage and capable of maintaining an lubricating effect during that time (period); and
(4) capable of preventing corrosion of the magnetic layer for a long period of time.
The lubricant such as the higher fatty acid, the ester thereof or the like which has conventionally been used is hardly said to fully satisfy such performances as described above and is unsatisfactory in practical characteristics such that, in a field of the magnetic recording medium, for example, a decline in a playback output level is caused by an insufficient ability of the lubricant to be used at a shuttle traveling test.
Further, by adopting a ferromagnetic metallic thin film, it has become necessary to provide lubrication of high precision to sliding between a magnetic tape or a magnetic disc and a magnetic head. For example, in the magnetic tape or the magnetic disc, a lubricant layer is extremely thinly formed on a surface of a magnetic layer in order to realize a high output by minimizing a loss of spacing between the magnetic recording medium and the magnetic head while securing durability and reliability of a vapor-deposited tape or hard disc. As for materials to form such lubricant layer as described above, it is required that a material having an excellent lubricating property is developed.
As for lubricants for metallic thin film type magnetic recording medium, a monocarboxylic acid having a fluoroalkyl ether group, a monocarboxylic acid having a perfluoropolyether ester structure or the like is proposed.
In Patent Document 1 to be described below, as for the lubricant for the metallic thin film type magnetic recording medium, a fluorine-containing alkyl succinic acid diester as shown in the following general formula (9) is proposed; however, this fluorine-containing alkyl succinic acid diester has a problem in that it has a large friction coefficient:R1—CH(COOR2)CH2COOR3   (9),
wherein R1 represents an aliphatic alkyl group or aliphatic alkenyl group; and
one of R2 and R3 represents a fluoroalkyl ether group while the other one of R2 and R3 represents any one of a fluoroalkyl group, a pluoroalkenyl group, a fluorophenyl group, an aliphatic alkyl group and an aliphatic alkenyl group.
In Patent Document 2 to be described below, esters of a fluorine-containing hydrocarbon dicarboxylic acid are proposed as lubricants as shown in following general formulae (10) to (12); however, since a compound as shown in the general formula (10) has only one carboxyl group having a large polarity and, in compounds as shown in the general formulae (11) and (12), a polar group is only an ester bond, all of these compounds have a problem in that they have a large friction coefficient:

wherein RA and RB each represent a hydrocarbon group; and
n≧3 and 0≦m≦5.
In Patent Document 3, a dicarboxylic acid containing organic groups A1 and A2 as shown in the following general formula (13) and having two ester bonds as shown in the following general formula (14) is proposed; however, there is a problem in that, since a hydrocarbon chain exists in each of terminals, fluorination effect is small and, accordingly, durability of a shuttle is deteriorated:

wherein Rf represents a fluorine-containing hydrocarbon group;
R1 represents a hydrocarbon group; and
R2 in one of A1 and A2 represents a hydrocarbon group while
R2 in the other one of A1 and A2 represents a hydrogen atom or a hydrocarbon group.
Patent Document 1: JP-A No. 6-41561 (pp. 3 to 5);
Patent Document 2: JP-B No. 8-16979 (pp. 2 and 3); and
Patent Document 3: JP-A No. 2001-55593 (pp. 2 to 4).